1. Field of the Invention
The present invention relates to an ultrasonic signal focusing method for focusing a received ultrasonic signal reflected from an object, and more particularly, to an ultrasonic signal focusing method for controlling a focusing delay time of a received ultrasonic signal to obtain a maximum resolution for use in an ultrasonic imaging system.
2. Description of the Related Art
In general, an ultrasonic imaging system emits an ultrasonic signal to an object to be examined, receives the ultrasonic signal reflected and returning from the discontinuous plane in the object, and then converts the received ultrasonic signal into an electrical signal to output it to a predetermined imaging apparatus, to thereby show the internal sectional structure of the object. The ultrasonic imaging system is widely used in a medical diagnostic field, a non-destructive testing field, an underwater detection field, etc.
In the ultrasonic imaging system, one of crucial factors required for functional improvement is an ultrasonic image resolution. It is steadily under development to improve the resolution. To improve the resolution, it is general that a recent ultrasonic imaging system uses an array transducer and performs transmission and receipt focusing through an electrical signal processing. A focusing method for an ultrasonic signal will be described below with reference to the accompanying drawings.
FIG. 1 shows transmission and receipt of an ultrasonic signal using an array transducer. The array transducer including a plurality of transducer elements converts an electrical signal into an ultrasonic signal and emits it to a focal point on an object. Then, the ultrasonic signal is reflected from a plurality of discontinuous planes on the object, and the reflected signal is input to the array transducer. When a plurality of discontinuous boundary planes exist in an object, an ultrasonic signal in each boundary plane is reflected in sequence and then input to the array transducer. The ultrasonic signal input to the array transducer after being reflected from the object has a different arrival of time according to the location of each transducer element. As shown in FIG. 1, a transducer element #0 located in the center of the array transducer receives an ultrasonic signal travelling as far as a distance of S.sub.0 and being reflected from a focal point. However, since the n-th transducer element #n receives an ultrasonic signal travelling as far as a distance of S.sub.n (S.sub.n =S.sub.0 +.DELTA.S.sub.n) and returning therefrom, an arrival of time at the n-th transducer element #n is delayed as much as time corresponding to a distance of .DELTA.S.sub.n compared to the central transducer element #0. That is, as a transducer element is farther from the central transducer element #0, a time taken until when an ultrasonic signal arrives at the transducer element is prolonged. As described above, the ultrasonic signals input at a respectively different time are converted into an electrical signal in each transducer element. Thus, the above time difference should be delayed and compensated for in order to perform a focusing of the electrical signal output from each transducer element.
FIG. 2 shows a receipt focusing at the time of receiving an ultrasonic signal. The ultrasonic signals input to the transducer elements in the array transducer are applied to delays in sequence of time when the ultrasonic signals arrive at the transducer elements. Each delay delays an ultrasonic signal by a time difference corresponding to a distance where the firstly applied ultrasonic signal has proceeded, and outputs the delayed result. Thus, as shown in FIG. 2, the phases of the ultrasonic signals having passed through the delays are aligned in a line. An adder adds all the ultrasonic signals whose phases have been aligned in a line. Then, it becomes as if the ultrasonic signals having started at the focal point arrive at all the transducer elements at the same time. Since these signals are same in phase, the amplitude of the ultrasonic signal becomes maximized at the point where the ultrasonic signals are added. However, at the points other than the point where the ultrasonic signals are added, since the signals do not arrive at the same time, their phases are different from each other and offset resulting in a weak signal.
The above delay will be described in more detail using the following equation (1). Assuming that a distance between the central transducer element #0 and the transducer element #n in the array transducer is X.sub.n at the time of focusing the ultrasonic signal, an arrival delay distance .DELTA.S.sub.n is calculated as the following equation (1). EQU .DELTA.S.sub.n =S.sub.n -S.sub.0 =S.sub.0.sup.2 +x.sub.n.sup.2 +L -S.sub.0 (1)
Here, S.sub.n represents a distance from the focal point to the transducer element #n, and S.sub.0 represents a distance from the focal point to the transducer element #0.
Also, an arrival delay time .DELTA.td.sub.n of the n-th transducer element #n with respect to the central transducer element #0 is calculated as the following equation (2). EQU .DELTA.td.sub.n =.DELTA.S.sub.n /C.sub.0 (2)
Here, C.sub.0 represents an ultrasonic signal travelling velocity in the medium including an object. Thus, if the number of the transducer elements is (2N+1), a focusing delay time of the n-th transducer element #n is calculated as the following equation (3). EQU .DELTA.fd.sub.n =.DELTA.td.sub.N -.DELTA.td.sub.n (3)
When the focusing delay time .DELTA.fd.sub.n is applied to the n-th transducer element #n, the phases of the signal can be aligned in a line as shown in FIG. 2. Here, a curve formed by connecting the focusing delay times with respect to the signals received at all the transducer elements is called a focusing time delay curve.
An ultrasonic signal travelling velocity (the velocity of sound) used for calculation of the focusing delay time in the current ultrasonic imaging system as described above uses a value of 1540 m/s which is an average velocity at a soft tissue of a human body. The human body is formed of a composite medium having various velocities from 1400 m/s to 1600 m/s, among which fat having a velocity of 1400 m/s becomes the greatest error factor. In particular, in the case that subcutaneous fat is thick at the time of abdominal diagnosis, the actual time taken until when the signal is returned is lagged in time compared to the arrival time calculated under the assumption of a uniform velocity. The above error effect causes reduction of an image brightness, lowering of a resolution, deformation of a shape, a ghost phenomenon, etc., due to a decrease of a main lobe and an increase of a side lobe in an ultrasonic signal because the phase of the received signal is not aligned thereby lowering a focusing characteristic. In addition, a big error is brought about in the case of application such as the calculation of a capacity of the heart or kidney requiring geometrical size or distance information of the medium. In order to compensate for the above error, an error of a focusing delay time generated due to a difference between the ultrasonic signal travelling velocities at the media or a relative velocity difference depending upon paths is obtained and then offset. However, it has been difficult that an ultrasonic signal travelling velocities at the media and a relative velocity depending upon the travelling path are obtained in each transducer element.